Played By: Megan Fox
- Alas, Poor Villain: The fact that Jennifer is possessed shows she's not in complete control of her actions and by doing the ritual on her, Low Shoulder is pretty much responsible for her death.
- Alpha Bitch: She's a popular cheerleader who's also a massive bitch, to the point where the other students consider her "evil." Despite this, guys trip over themselves to impress her. She does have shades of a nicer side, at least at first, but only to Needy.
- Ambiguously Bi: Much like Needy, she shows many traits of this, actually trying to (and succeeding at) seduce her at one point. She also utters this great line:Needy: I thought you only killed boys!
Jennifer: I go both ways.
- Deconstructed by Word of God. Megan Fox says that she played Jennifer as a deeply closeted lesbian.
- Berserk Button: Don't tell her about how she had to use laxatives to stay skinny back when she was alive.
- Big Bad Friend: She was Needy's Childhood Friends and Best Friend before she got possessed..
- Blessed with Suck: Unlike Needy, who is Cursed with Awesome, Jennifer enjoys her powers but she's also a mangled, evil demon that needs to feed on others to survive.
- Brainy Brunette: Inverted. She's your typical vapid cheerleader.
- Breast Attack: Needy attacks her breasts to get to her heart.
- The Cheerleader: The vapid, skanky type, and that's before she gets possessed.
- Cursed with Awesome: She seems to think so after becoming possessed by the demon, but in reality it's Blessed with Suck.
- Decoy Protagonist: Her name may be in the title, but the story is about Needy.
- Depraved Bisexual: Possibly. She "go[es] both way" by her own admission, but Megan Fox has stated that she viewed Jennifer as gay. But she still kills and eats people, and even before that, she was a Manipulative Bitch who bullied Needy.
- Drunk on the Dark Side: Jennifer really enjoys her new powers after becoming possessed and seems to revel in the murders she commits.
- Enemy to All Living Things: Subverted. The animals are drawn to her.
- For the Evulz: When she could've eaten any other guy, she killed Chip just to hurt Needy.
- Fille Fatale: It can be easy to forget because of Dawson Casting, but Jennifer is in her last year of high school and extremely sexually active, including with adult men.
- Friendly Enemy: Zigzagged towards Needy, kind of. But Needy soon figures out that this is at least partially a pretence because Jennifer still needed to feel superior so she could hurt Needy more.
- Game Face: Jennifer's face transforms into her new demonic visage when she's feeding, or when she's trying to intimidate someone.
- A God Am I: She tells Needy this when defending her current situation. After realizing her newfound healing ability, Jennifer burns her tongue with a lighter, then loudly declares "I am a god."
- Have I Mentioned I Am Heterosexual Today?: It's heavily implied that the reason she sleeps around so much and flirts with boys is to cover up her own feelings for Needy.
- Healing Factor: Jennifer can regenerate from any injury, she recovered from being impaled through the stomach, having her tongue burned and having her arm cut with a needle.
- Horny Devil: Jennifer turns into a type of succubus after she's sacrificed by Nikolai.
- Horror Hunger: She vomits a trail of black, spiny fluid if she eats anything but men. But she does adapt to her new diet very well.
- Iconic Outfit: Jennifer likes to sleep in a 3/4-sleeve shirt, short underwear, and either knee-length socks or legwarmers.
- I Have Boobs, You Must Obey!: She uses her assets to get drinks despite being underage, and believes that breasts are like smart bombs that give women power.Jennifer:(when asked how she will get alcohol) Uh, I'll just play hello titty with the bartender.
- I'm a Humanitarian: Post-possession, she has to eat the people she kills. If her attempts at eating a roast chicken are any indication, she literally can't eat anything else.
- Jerkass: More so in the post demonic possession.
- Literal Maneater: She seduces men, luring them in with the promise of sex... and then kills them and eats their remains.
- Ms. Fanservice: She wears very revealing clothing through the movie, usually baring her legs and acting very sexually aggressive. She also has a Skinny Dipping scene with a shot of her surfacing from the water naked, although the scene combines Foot Focus, Shoulders-Up Nudity and Toplessness from the Back to hide any of her naughty bits.
- Popular Is Dumb: The most popular girl in school, who isn't very smart.
- Psycho Lesbian: Another possible interpretation of Jennifer's behavior. While she does seduce (and kill) boys, she is really only attached to Needy, and her flirtation and hooking up with Needy is contrasted with the lack of enjoyment she gets from boys.
- Really Gets Around: Jennifer's has been sleeping around since Junior High. When Low Shoulder assume she's a virgin and try to use her for a Virgin Sacrifice, it turns her into a Literal Maneater Horny Devil.
- Romantic Two-Girl Friendship: She's in one with Needy, but it's a little bit more twisted than usual.
- Self-Mutilation Demonstration: Jennifer burns her own tongue with a cigarette lighter, and slashes herself in front of Needy when she decides to share her 'secret' with her.
- She's Got Legs: Most of the movie posters focus on Jennifer's legs.
- Skinny Dipping: After eating Jonas, Jennifer goes for an after meal swim in the lake while naked, presumably to wash off all the gore from her body.
- Slasher Smile: Jennifer displays one at various times. Most notably after she first becomes possessed and when she appears in Needy's vision sitting on top of the chair watching her and Chip have sex.
- Straw Feminist: Jennifer has a bit of this trait, claiming that PMS was invented by "boy-run media to make (women) seem crazy". An unused exchange between her and Needy also has her trying to justify her murders thus:
- Needy: You're killing people!
Jennifer: No, I'm killing boys.
- Villain Protagonist: She's the film's main character, a Literal Maneater that wants to devour every boy in her school.
Anita "Needy" Lesnicki
Played By: Amanda Seyfried
- Ambiguously Bi: She has a boyfriend, whom she cares about very deeply. She also makes out with Jennifer, and seems to be really into it before she breaks it off.
- Berserk Button: Don't tell her what to eat. An orderly lost a tooth when she recommended she eat more than Toastems.
- Break the Cutie: She starts as a kind but insecure Shrinking Violet, then has to deal with her best friend transforming into a monstrous demon, people around her dying, losing her boyfriend, then finally having to kill Jennifer and being locked up in a mental ward.
- The Cassandra: She tries to tell everyone over and over that Jennifer is a monster, both before and after her death, and even after that, she tries to convince the psych ward staff about the devil sacrifice. Nobody believes her.
- Cool Loser: She's supposed to be the "ugly best friend", but is clearly drop dead gorgeous and best friends with Jennifer, the most popular girl in school. It's later shown that she downplays her own looks and covers herself up so as not to outshine Jennifer.
- Cursed with Awesome: When Needy kills Jennifer, she gains some of her powers, which boils down to: all the good ones, none of the bad. However, unlike Jennifer, Needy doesn't relish them but uses them to get her job done.
- Dumb Blonde: Inverted. She's incredibly smart, in contrast with her dumb popular friend Jennifer.
- Hollywood Homely: Invoked. Needy has to be more plain around Jennifer.
- Narrator: She's the story's narrator
- Nerd Glasses: She wears a very unflattering pair of nerdy glasses.
- Only Known by Their Nickname: No one ever calls her "Anita."
- Roaring Rampage of Revenge: She slaughters Low Shoulder at the end of the movie for what they turned Jennifer into.
- Shipper on Deck: She ships Jennifer/Colin, and encourages Jennifer to go out with him. This doesn't end well.
- Supporting Protagonist: She's The Hero of the film, but still takes a secondary role to Villain Protagonist Jennifer.
- Took a Level in Badass: After gaining part of Jennifer's powers, she breaks out and massacres all of Low Shoulder.
- Took a Level in Jerkass: She becomes extremely violent and unstable after being well and truly broken. The How We Got Here opening has her kicking an orderly (whose only "crime" was to chastise Needy for her choice of food) so hard she knocks one of her teeth out.Needy: I recommend that you shut the fuck up!
Played By: Johnny Simmons
- Character Death: The last of Jennifer's victims.
- Clingy Jealous Guy: He's very possessive of Needy, being jealous of her relationship with both Jennifer and Colin.
- Died in Your Arms Tonight: He dies in Needy's arms after Jennifer tore his neck open.
- Dogged Nice Guy: A positive version, but he is very possessive of Needy and jealous of her friendship with Colin.
- Dying Moment of Awesome: Chip's impalement of Jennifer.
- Face Death with Dignity: He impales Jennifer, which fails to kill her, but he still gets serious points for trying, and then, as he lies dying, he uses his last moments to try and comfort Needy, telling her that he loves her, and peacefully accepting that it's his time to go.
- Nice Guy: He's kind and supportive of Needy, and seems to get along well with everyone except Jennifer.
- Sacrificial Lion: His death is what makes Needy finally, really commit to killing Jennifer for once and all.
Played By: Adam Brody
- Greater-Scope Villain: His Human Sacrifice of Jennifer made her what she became.
- Hoist by His Own Petard: Killed by Needy with the same knife he used to sacrifice Jennifer.
- Jerkass: Even beyond his obvious evil deeds of killing an innocent girl for a record deal, he and his band go out of their way to mock Jennifer as she's dying, seemingly for no other reason than they feel like it.
- The Leader: Of his indie rock band, "Low Shoulder".
- Names to Run Away from Really Fast: Who would've guessed that "Nikolai Wolf" is a villain?
- Religion of Evil: He and his band are Satan worshipers.
- Villain with Good Publicity: The band became local heroes in town for "helping people" escape the fire at the bar, despite them being the ones that caused it in the first place.
- Virgin Sacrifice: A Satan worshiper that was looking for a virgin to sacrifice to Satan so he and his band could cut a record deal.
Played By: Kyle Gallner
- Character Death: He ends up as the third of Jennifer's victims.
- Emo/Goth: He has the hairstyle and wears makeup and has piercings. Needy describes him as "dark and emotional".
- Nice Guy: Despite his goth sensibilities, he's very friendly, and is good friends with Needy.
Played By: J. K. Simmons
- Artificial Limbs: He has a gnarly, robotic arm.
- Stern Teacher: The stern science teacher at Devil's Kettle school.